1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method, and more particularly, to an image forming apparatus having large nozzles and small nozzles which eject liquid droplets of different volumes for the same color.
2. Description of the Related Art
An inkjet recording apparatus is known that is provided with large nozzles and small nozzles for ejecting droplets of liquid or ink of the same color and mutually different volumes to be deposited onto a recording medium to form high-quality images having high tonal graduation. The large and small nozzles eject the large and small droplets at a prescribed ratio in accordance with the image to be recorded. In a thermal jet method, which performs ejection by using heating elements, a composition including large nozzles and small nozzles is particularly beneficial, since it is difficult to achieve satisfactory control of the ejection of liquid droplets having different volumes from the same nozzle, in comparison with a piezoelectric method using piezoelectric elements as actuators.
Banding (e.g., white stripes) may occur in the recorded image due to errors in the positions and the sizes of the dots formed on the recording medium by the liquid droplets ejected from the respective nozzles. As a method of reducing the visibility of banding, Japanese Patent Application Publication No. 2004-148723, for example, discloses a method for arranging the dot pattern in such a manner that large and small dots formed by droplets ejected from large nozzles and small nozzles do not overlap with each other. Moreover, Japanese Patent Application Publication No. 2005-153435 discloses a method according to which large nozzles and small nozzles are alternatively arranged so that the intervals between large dots are covered over by small dots without leaving any spaces. However, these methods cannot be expected to yield sufficient beneficial effects if, for example, there are nozzles suffering ejection failure due to nozzle blockages, or other causes.
Japanese Patent Application Publication No. 2004-58284, for example, discloses a method for reducing the visibility of banding caused by nozzles suffering ejection failure, in which recording data corresponding to a nozzle suffering ejection failure (ejection failure nozzle) is distributed to the recording data corresponding to the nozzles positioned adjacently to the ejection failure nozzle (adjacent nozzles). However, in this method, a burden is placed on the adjacent nozzles to which the recording data of the ejection failure nozzle is assigned, and the nozzle life is shortened. Furthermore, in this method, a droplet is simply ejected to form a dot at a position adjacent to the originally intended dot formation position of the ejection failure nozzle (i.e., the banding position), and no droplet is thereby deposited to form a dot directly at the banding position. Consequently, there are limitations on the reduction of the visibility of banding, and it is difficult to improve image quality further.